ABSTRACT Evaluation of a Complex Behavioral Intervention for Young Adults with Diabetes: The Resilient, Empowered, Active Living-Telehealth (REAL-T) Study will address the unmet self-management and psychosocial needs of young adults (YAs) with type 1 diabetes (T1D). The purpose of REAL-T is threefold: (1) to evaluate the efficacy of the REAL intervention as administered via telehealth (REAL-T), in improving glycemic control (HbA1c) and psychosocial outcomes; (2) to examine the extent to which beneficial intervention effects are retained over a 6- month follow-up period; and (3) to investigate the mediating mechanisms responsible for the intervention's effects. Young adulthood is a challenging life stage for many with T1D; only 17% of YAs ages 18-25 and 30% ages 26-30 attain recommended HbA1c targets, and only 2% consistently perform self-care in accordance with national guidelines. Yet, it is a crucial stage for establishing health habits that persist throughout adulthood. Several issues impede diabetes care during this life stage: first, YAs transfer from pediatric to adult healthcare settings; second, their lives are characterized by frequent changes in lifestyle, roles, and routines, which make consistent self-care challenging; and third, one-third of YAs with T1D experience mental health challenges, and half have diabetes complications and medical comorbidities, which further complicate diabetes management. To address these issues, our research team developed REAL, a 6-month individually tailored occupational therapy intervention focused on incorporating diabetes self-care into participants' daily habits and routines, and evaluated its efficacy in a randomized controlled trial (n=81). REAL was shown in intention-to-treat analysis to significantly improve HbA1c (0.8% reduction; p=0.01) and diabetes-related quality of life (p=0.04). While REAL was highly promising in its impact on health and quality of life, the intervention was delivered through home visits, limiting its potential for broad dissemination. Given that our target population experiences significant logistical barriers to clinic attendance (the impetus for our in-home treatment model), we adapted REAL to be delivered via telehealth (REAL-T), which was shown in a proof-of-concept study to be feasible, highly acceptable, and has promise to induce similar health and quality life of life benefits as did REAL as delivered in person. We propose to conduct a large-scale RCT (n=240) to compare REAL-T to usual care in improving glycemic control (HbA1c and continuous glucose monitor-derived measures), psychosocial well-being, and hypothesized intervention mediators. In addition, we will perform health economic analyses to determine the extent to which REAL-T is cost-effective or produces cost savings. The study's specific aims are as follows: Aim 1: Evaluate the efficacy of REAL-T in improving glycemic control and psychosocial well-being. Aim 2: Assess the long-term carryover (3- and 6-months post-intervention) of beneficial effects of REAL-T on glycemic control and psychosocial well-being. Aim 3: Examine mediating mechanisms of the REAL-T intervention's effects on glycemic control and psychosocial well-being through structural equation modeling (SEM).